1. Field of the Invention
The present invention relates to an electric power control switch, and, in particular, to a variable electric power control switch for controlling electric power by varying the application time of current to an electric load in electric heating equipment such as an electric range or an electric oven.
2. Description of the Prior Art
Generally, an electric heating equipment comprising one or more electric heating units, such as an electric range or an electric oven, includes an electric power control switch adapted to control the heating temperature of each heating unit, depending upon the desire of the user.
The heating temperature of a heating unit can be regulated by controlling the electric power applied to the heating unit, that is, the application time of current thereto.
Conventionally, a device utilizing a bimetal, a so called "infinite switch", has been used as an electric power switch of the above-mentioned type wherein the temperature of heating unit is constantly regulated by varying the application time of current to the heating unit. Such an infinite switch utilizes a bimetal including a heater, so that contacts on the bimetal repeat ON and OFF conditions in accordance with the heating temperture of the heater. In this arrangement, the period of repeating ON and OFF conditions of the contacts greatly varies due to the variation of temperature around the bimetal, so that the operater can not control constantly and accurately the electric power as desired. In view of the fact that in this switch contacts change ON and OFF conditions thereof, depending upon expanded and contracted deformation of bimetal, opening and closing of contacts can not be effected in a moment. Furthermore, severe spark is generated in the opening and closing of contacts, thereby causing contacts to be severely worn. After being used for a long time, the gap between contacts varies due to the wearing of contacts, so that the period of opening and closing the switch varies, thereby causing the accuracy of the switch to be decreased. The moveable member of the switch has to be made of expensive material having high elasticity, in order to maintain sufficient gap between contacts to prevent the generation of spark in opening of contacts. In addition, the elastic fatigue of the moveable member is increased due to the large deformation of said moveable member, thereby causing the life of the switch to be shortened.
Other than the above-mentioned infinite switch utilizing a bimetal, there is an electric power control switch wherein a contoured cam driven by a motor serves to periodically urge a moveable contact blade against a fixed contact blade, in order to open and close contacts on said blades, respectively. For example, a defrost timer for a refrigerator, a timer for a washing machine, or a rhythm timer for a motor fan have been known as the above-mentioned type of electric power control switch. In such motor-driven switch, an overload is repeatedly subjected to the moveable contact blade, thereby causing the life of the switch to be shortened. As in the case of the infinite switch, there is a problem of severe wearing of contacts caused by a generation of spark between contacts, so that the period of repeating ON and OFF conditions of contacts greatly varies, thereby causing the accuracy of the switch to be lowered.